Used with the 3T7, a TDR system is formed. The 3S7/3T7 are seen on the front cover of the document
Tektronix TDR Measurement Concepts.
Like the 1S2, the connectors on the 3S7/3T7 are 50 Ω GR-874 connectors.
A short coaxial cable carries the pulse from the front panel output of the
3T7 to the front panel input of the 3S7, where it passes through the
sampling gate and out to the cable or network under test.

The interface between the GR-874 connector and the printed circuit board is described in US Patent 3,426,311.